The present invention relates to chemical/biological environmental sensors. More specifically, the invention relates to systems and methods for smart FAIMS sensors that dynamically change their operating point in response to their environment.
Field Asymmetric Ion Mobility Spectroscopy (FAIMS) may be used to separate molecular or atomic ions based, in part, on the ions nonlinear ionic mobility in an electric field. In a typical FAIMS configuration, ions are directed between two plates that generate an electric field perpendicular to the flow direction of the ions. The electric field may be generated by applying a time varying voltage to the two plates. The time varying voltage is usually a superposition of two time varying signals or a superposition of a time varying signal and an adjustable constant signal.
A first component of the time varying signal is an asymmetric oscillation wherein the peak magnitude of, for example, the positive portion of the oscillation is different from the peak magnitude of the negative portion of the oscillation. The absolute value of the magnitude of the asymmetric signal is such that the electric field generated is usually greater than about 5,000 V/cm during the positive portion of the oscillation and less than about 1,000 V/cm during the negative portion of the oscillation cycle. In the example above, the durations of the positive and negative portions of the cycle may be adjusted such that the products of the electric field and the duration are approximately the same for both the positive and negative portions of the oscillation. In the example above, the duration of the negative portion of the oscillation cycle is preferably five times longer than the duration of the positive portion of the oscillation cycle.
If the ionic mobility of the ion is independent of the applied electric field, the ion will oscillate transversely to its direction of travel but will not drift transversely to its direction of travel. The ionic mobility, however, is usually not independent of the applied electric field and the ion will drift toward one of the electrodes and transversely to its direction of travel, the direction of the drift depending on whether the ionic mobility is an increasing or decreasing function of the applied electric field. If uncompensated, the ion will continue to drift toward one of the electrodes until it collides with the electrode.
A second voltage signal, VC, may be superposed onto the oscillating signal to compensate the transverse drift of the ion. The transverse drift depends, inter alia, on the ion mass and the ion mobility, which are usually unique to each ionic species. By adjusting the second voltage signal to cancel the transverse drift of the ion, herein referred to as a compensation voltage, an operator of the device may select a particular ionic species. Alternatively, by sweeping the second voltage signal, the operator may obtain a spectrum of ionic species ordered by the combination of the species' mass and mobility.
Ions may be directed between the electrodes by a pump or by a second set of electrodes that generate an electric field in the direction of the ion's flow path. For example, U.S. Pat. Nos. 6,495,823 and 6,512,224 issued to Miller teach the use of a mechanical pump or a pair of electrodes to direct ions between the electrodes generating the transverse electric field. The use of a mechanical pump, however, has several disadvantages when FAIMS is used as a sensor. The mechanical pump usually adds significant bulk to the sensor and requires large power relative to the sensor. Furthermore, the time response of the mechanical pump significantly increases the time response of the FAIMS sensor. The electrical pump disclosed by Miller also adds to the bulk of the sensor system by the addition of the second pair of electrodes and its associated electronics.